1. Field of the Invention
The invention relates generally to downhole tools which incorporate one or more atmospheric chambers.
2. Description of the Related Art
A number of downhole tools rely upon the hydrostatic pressure of the wellbore in order to be actuated. These downhole tools include packers and locks that are set using hydrostatic pressure. Typically, such tools incorporate at least one collapsible atmospheric chamber into their setting mechanism. When the tool is constructed at the surface prior to being run into the well, the atmospheric chamber is usually enclosed and sealed off so that it contains an amount of fluid (usually air) at atmospheric pressure.
During setting of the tool, the atmospheric chamber is collapsed due to a pressure differential between the atmospheric chamber and surrounding hydrostatic and applied pressure within the wellbore. Following collapse, the atmospheric chamber continues to retain a small amount of fluid. A significant pressure differential between the atmospheric chamber and surrounding pressure is desirable to ensure positive actuation of the tool. The inventors have recognized, however, following actuation of the tool, a significant pressure differential can act to reduce the rating of the tool.